The Hi Fi is a great venue, as the area around the mosh pit is elevated a couple of steps so you can see the stage really well.
As for the 'no seat' policy, that's crap because like the other guy said, I've seen a comedy performance there and the place was full of seats. It had a great atmosphere as well. If Opeth wanted seats (and I doubt they would) it would be quite easy to have them.
The Hi Fi bar is the best venue in Melbourne in my opinion... Great mosh, good side vision, plenty of room at the back with the bar... it's great.